Recently, coordinated transmission from multiple radio network nodes, such as radio base stations, to at least one mobile terminal, or Coordinated Multi-Point (COMP) transmission, has been shown to provide performance gain as compared to without coordination. However, in the downlink, the coherent coordination needed for coordinated transmission requires Channel State Information (CSI) at the transmitter side, i.e. at the radio base stations. It is quite challenging to provide the transmitter side with channel station information, at least from a system-design point of view.
Also, in a Multi-User Multiple-Input Multiple-Output (MU-MIMO) system, in which capacity of the downlink channels, this kind of channel state information may be useful for improving capacity. Several solutions, as discussed in for example, “On the capacity of MIMO broadcast channels with partial side information”, IEEE Trans. on Inform. Theory, vol. 51, no. 2, pp. 506-522, February 2005, by M. Sharif and B. Hassibi, and “MIMO broadcast channels with finite rate feedback”, IEEE Trans. Information Theory, Vol. 52, No. 11, pp. 5045-5059, November 2006 by N. Jindal, for attacking this problem have been proposed. These solutions focus on systems, in which a base station antenna is placed at a fixed point. As a result, channel response between transmit and receive antennas may be assumed to be Independent and Identically Distributed (IID). Based on this assumption, random beam forming and random vector quantization have been proposed to reduce size expressed as number of bits of the CSI. The solutions presented in these documents may, however, not be applied to a coordinated multi-point transmission system. Hence, there is a need for a channel state information feedback procedure which may be applied to a coherent coordinated multi-point transmission system.